Long-Duration Energy Storage

Long-Duration Energy Storage (LDES) refers to a range of technologies capable of storing energy for more than eight hours and, in some cases, for days, weeks or seasons. These technologies can support energy shifting, congestion management, balancing services and the integration of larger shares of renewable energy into the power system.

The process is simple: 

Long-Duration Energy Storage works by taking electricity when it is available in excess, converting it into another form of energy, storing it, and converting it back into electricity when the power system needs it.

In practice, this happens in three steps:

• Charging: surplus electricity is used to compress air, liquefy air or carbon dioxide, pump water uphill, or produce hydrogen.
• Storage: the energy is kept in a storage medium, such as compressed air, liquid air, liquid carbon dioxide, water in an upper reservoir, or hydrogen.
• Discharging: when electricity is needed, the stored energy is converted back into power through pump-turbines, expanders, gas turbines, generators or other power conversion equipment.

This makes LDES different from short-duration storage: it is designed not only to respond quickly, but also to deliver electricity over longer periods, from several hours to multiple days, and in the case of hydrogen, potentially weeks or seasons. In this way, LDES helps bridge the gap between renewable electricity generation and demand over several hours, days or even longer periods.

Its main components: 

• Power conversion equipment: This includes motors, generators and electrical systems that allow the storage plant to absorb electricity during charging and deliver electricity during discharge.
• Compressors: These are used in several storage technologies to pressurise air or carbon dioxide. During charging, compressors convert electricity into stored mechanical or thermodynamic energy.
• Pumps and pump-turbines: Pumps move fluids such as water, liquid air or liquid carbon dioxide. In pumped storage hydropower, pump-turbines move water uphill during charging and generate electricity when the water flows back down during discharge.
• Expansion turbines: These turbines recover stored energy by expanding air or carbon dioxide and converting it into mechanical power that drives a generator.
• Generators: These convert the mechanical energy produced by turbines or pump-turbines back into electricity.

Compressors, turbines and pump-turbines are therefore key enabling components for several LDES technologies. They make it possible to store electricity when it is abundant and recover it when the power system needs flexibility.

Same purpose, different storage media

LDES technologies serve the same broad objective, but they store energy in different ways. Some store pressure, some store cold or heat, some store gravitational potential energy and some store chemical energy.

LDES technologies come in various types: 

• Compressed Air Energy Storage (CAES): it is a scalable solution for medium- to long-duration energy storage that uses surplus electricity to compress air and later expanded through a turbine to generate electricity. It notes that system performance depends on multi-stage compression and expansion, thermal management and the characteristics of the storage reservoir. CAES as one of the most promising long-duration storage technologies expected to be deployed over the coming years.
• Liquid Air Energy Storage (LAES): stores electricity by liquefying air at very low temperatures and storing it in cryogenic tanks. During discharge, the liquid air is evaporated, pressurised and expanded through a turbine to produce electricity. It is suitable for locations without the geological requirements of cavern-based storage.
• Liquid CO2 Energy Storage (LCES): uses carbon dioxide in a closed thermodynamic cycle combining compression, liquefaction, thermal management and expansion.
• Pumped Storage Hydropower: stores energy by moving water between reservoirs at different elevations and remains the backbone of large-scale storage in the EU.
• Hydrogen-to-Power: uses electricity to produce hydrogen, stores it, and converts it back into electricity when needed, including for seasonal flexibility.

Several main configurations: 

• Daily and multi-hour storage: used to shift solar generation into evening peaks or manage intraday renewable variability.
• Multi-day storage: used to bridge longer periods of low power generation.
• Seasonal storage: mainly associated with hydrogen, where energy can be stored over much longer periods.
• Site-dependent storage: technologies such as pumped storage hydropower and some CAES projects depend strongly on geography or geology.
• Site-flexible storage: technologies such as LAES and some engineered CAES or LCES concepts can be located closer to demand centres, renewables or industrial sites.

LDES has different applications to support the energy transition: 

All in all, LDES technologies can:

• Shift electricity across multiple hours or days.
• Reduce renewable energy curtailment.
• Support congestion management.
• Provide balancing and ancillary services.
• Improve the integration of renewable energy into the electricity system.
• Enhance security of supply during periods of low renewable generation.

Technology overview 

TECHNOLOGY	EUROPEAN RELEVANCE	STORAGE MEDIUM	TURBOMACHINERY ROLE
CAES	Operational experience in Germany and renewed interest in advanced European projects	Compressed air	Compressors and expansion turbines
LAES	Locatable long-duration storage option for grids and industrial sites	Liquid air	Compressors, cryogenic pumps and expansion turbines
LCES	Emerging thermodynamic storage concept for utility-scale flexibility	Liquid/compressed CO2	Compressors, pumps and turbines
Pumped Storage Hydropower	Main large-scale storage method in the EU and a key provider of flexibility	Water elevation difference	Pump-turbines
Hydrogen-to-Power	Relevant for very long-duration and seasonal storage	Hydrogen	Hydrogen-capable gas turbines or other reconversion equipment